Improving the Robustness of Organic Semiconductors through Hydrogen Bonding

Molecular organization plays an essential role in organic semiconductors since it determines the extent of intermolecular interactions that govern the charge transport present in all electronic applications. The benefits of hydrogen bond-directed self-assembly on charge transport properties are demonstrated by comparing two analogous pyrrole-based, fused heptacyclic molecules. The rationally designed synthesis of these materials allows for inducing or preventing hydrogen bonding. Strategically located hydrogen bond donor and acceptor sites control the solid-state arrangement, favoring the supramolecular expansion of the π-conjugated surface and the subsequent π-stacking as proved by X-ray diffraction and computational calculations. The consistency observed for the performance of organic field-effect transistors and the morphology of the organic thin films corroborate that higher stability and thermal robustness are achieved in the hydrogen-bonded material.


Theory and computational results
Marcus-Levitch-Jortner (MLJ) Rate Constant and its Parameters. Within a hopping mechanism, a MLJ rate constant has been selected since the MLJ rate incorporates quantum tunneling effects through an effective vibrational normal mode coordinate. The non-adiabatic electron-transfer MLJ rate is expressed as follows: the Boltzmann probability that a vibrational state n on an initial site i is occupied at a certain temperature.

S5
The transfer integral t ij (also known as electronic coupling) have been calculated according to the approach developed by Baumeier et al., 2 in which the molecular orbitals of a dimer are projected on the basis of the molecular orbitals of the individual molecules.
To take into account the impact of the thermal motions on t ij , molecular dynamics (MD) simulations (10 ps of equilibration and 100 ps of production) of cubic cells of 91 molecules for ADI and 125 molecules for ADAI were carried out at the GFN-FF level, 3 where the molecules in the edge of the crystal were kept frozen since periodic boundary conditions are not implemented in the xTB code. From the production simulation, 1000 snapshots with timesteps of 0.1 ps were extracted. For each snapshot, the transfer integrals t ij for the most relevant dimers were calculated at the B3LYP/6-31G** level using a homemade program. Relevant dimers were considered to be those that displayed significant couplings (t ij > 1 meV) at the experimental crystal structure (Table S1).
Average transfer integral values ( ij t ) and their corresponding standard deviations ( ij t  ) have been collected in Table S1.
The reorganization energy ( ) is a key parameter for the evaluation of electron transfer rates and is associated with the energy change owing to electronic redistribution and nuclear rearrangement in electron-transfer events. 4 Generally,  is split into the internal and external reorganization components . The former is related to the energy cost due to the intramolecular nuclear relaxation of the molecular systems during the electron-transfer reaction whereas the latter comes from the environmental effects; i.e., polarization and reorientation of neighboring molecules as a response of the charge (electron or hole) injection in a molecular system.
The internal reorganization energy  i n t has been estimated by using the four-point approach. 4 Table S1). H atoms have been omitted for simplicity.  Table S1). H atoms have been omitted for simplicity. Dotted blue lines indicates the molecules that form H-bonds. Figure S6. a) 2D map of t ij (in meV) for a face-to-face π-stacked dimer of ADAI calculated by moving one of the molecules along the x and z directions starting from the crystal structure (x = 0 Å and z = 0 Å). b) Topologies of the HOMOs for a face-to-face π-stacked dimer of ADAI at the crystal structure (top and bottom molecules are represented with bright and faded colors, respectively).  (Figure S4 and S5). t ij corresponds to transfer integrals computed at the experimental crystal structure. Transfer integrals were computed at the B3LYP/6-31G** level. The t ij values for the face-to-face π-stacked dimers in both ADI and ADAI are highlighted in blue. Average values and standard deviations obtained from six devices.